System and Method of Polishing a Surface

ABSTRACT

A method of polishing a surface of an object disposed within a gas chamber is provided. The method includes filling the gas chamber with a discharging medium to a predefined pressure, applying a voltage between an electrode and the surface, calibrating a height of the electrode relative to the surface so as to establish electrical breakdown threshold criteria, and scanning the electrode with respect to the surface so as to sequentially position the electrode over a plurality of locations on the surface, each location characterized by a surface error. When a respective location in the plurality of locations has a surface error that meets the electrical breakdown threshold criteria, electrical breakdown occurs, whereby the electrical breakdown results in a discharging pulse that polishes the surface.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application61/879,080, “System and Method of Polishing a Surface,” filed Sep. 17,2013, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

This present invention relates generally to a method and apparatus forpolishing a surface and, more particularly, to a method and apparatus ofpolishing a surface using a fast electric discharge.

BACKGROUND

Many semiconductor devices and integrated circuits are manufacturedusing photolithographic processes. Often in such processes, a circuitdesign is transferred from a photomask (sometimes called a reticle) to asemiconductor wafer by using an optical imaging scanner/stepper system(“scanner system”). High production yield and quality of the producedcircuits relies not just on the precision of the scanner system but alsoon the quality of the photomask and semiconductor wafer being used.Although many error sources influence the quality of the photomask andsemiconductor wafer, key among them is surface flatness and roughness ofboth the photomask and the semiconductor wafer.

The issue of surface flatness and roughness for photomasks andsemiconductor wafers is important in both the near future, asconventional optical lithography is pushed to its physical limits (e.g.,the diffraction limit), as well as for future unconventionallithographic processes that are needed to move beyond the limitations ofconventional optical lithography. For example, extreme ultraviolet(EUV)/soft x-ray lithographic processes are being explored as one way ofreplacing or supplementing conventional optical lithography. In EUV/softx-ray lithography, non-axially symmetric imaging layouts are frequentlyemployed. Such non-axially symmetric imaging layouts place tightrestrictions on error sources such as imaging aberrations, distortionsand overlay errors that, in turn, increase the surface flatness androughness requirements of the optical components (including the reticle)and the wafer into the range of nanometers.

Currently, surface polishing in the semiconductor industry is mainlyachieved by chemical mechanical planarization/polishing (CMP). Due totechnical limitations of this approach, it is likely limited in utilityto fabricated devices that can tolerate peak-valley flatness excursionson the order of micrometers or slightly better. Moreover, CMP is aglobal polishing technique, meaning that an entire chip or wafer istypically polished at once. As such, CMP is known to generate asignature “global bow” and create new surface errors while minimizesothers. Other available surface polishing methods (e.g., mechanicalpolishing, magneto-fluid, or ion beam bombardment methods, etc) aretedious and time consuming, and are not easily transferred tovolume-level production environments.

Therefore, what is needed is an improved polishing method that does notsuffer from the problems associated with chemical mechanicalplanarization/polishing while still providing acceptable throughput andhigh accuracy surface polishing.

SUMMARY

To address the aforementioned problems, some implementations provide amethod of polishing a surface of an object disposed within a gaschamber. The method includes generating a pixel map of the surface. Thepixel map includes a plurality of pixels including a first pixel and asecond pixel. The first pixel corresponds to a first surface errorassociated with a first location on the surface and the second pixelcorresponds to a second surface error associated with a second locationon the surface. The method further includes filling the gas chamber witha discharging medium to a predefined pressure, positioning an electrodewith respect to the surface such that the electrode is proximal to thefirst location, and determining if the first surface error meetspredefined polishing criteria. In accordance with a determination thatthe first surface error meets the predefined polishing criteria, themethod includes triggering an electrical breakdown of the dischargingmedium whereby the electrical breakdown results in a discharging pulsethat polishes the surface. In accordance with a determination that thefirst surface error does not meet the predefined polishing criteria, themethod includes forgoing triggering of the electrical breakdown of thedischarging medium, and re-positioning the electrode with respect to thesurface such that the electrode is proximal to the second location.

In some implementations, triggering the electrical breakdown includesapplying a voltage between the electrode and the surface. The voltage isgreater than a breakdown voltage of the discharging medium. In someimplementations, the application of the voltage between the electrodeand the surface is gated so as to control a temporal duration of thedischarging pulse. In some implementations, the application of thevoltage between the electrode and the surface is gated using agas-filled tube.

In some implementations, triggering the electrical breakdown includesapplying a preionization signal to a region between the electrode andthe surface. In some implementations, the preionization signal isprovided by one of a laser or an ultraviolet lamp.

In some implementations, the electrode is a needle-type electrode havinga tip. The tip has a distal end disposed proximal to the surface andcharacterized by a radius of curvature at the distal end within a firstpredefined range and an included angle within a second predefined range.In some implementations, the first predefined range is one of the groupconsisting of: 10 nm to 100 nm, 50 nm to 500 nm, and 100 nm to 2000 nm;and the second predefined range is one of the group consisting of: 15degrees to 20 degrees, 5 degrees to 45 degrees, and 10 degrees to 30degrees.

In some implementations, the pixel map is generated in real-time using asurface height measurement sensor. In some implementations, the pixelmap is generated using a metrology tool prior to the filling,positioning, and determining operations.

In some implementations, the electrode is a respective electrode in anelectrode array. The electrode array includes a plurality of electrodes.

To address the aforementioned problems, some implementations provideanother method of polishing a surface of an object disposed within a gaschamber. The method includes filling the gas chamber with a dischargingmedium to a predefined pressure, applying a voltage between an electrodeand the surface, calibrating a height of the electrode relative to thesurface so as to establish electrical breakdown threshold criteria, andscanning the electrode with respect to the surface so as to sequentiallyposition the electrode over a plurality of locations on the surface,each location characterized by a surface error. When a respectivelocation in the plurality of locations has a surface error that meetsthe electrical breakdown threshold criteria, electrical breakdownoccurs, whereby the electrical breakdown results in a discharging pulsethat polishes the surface.

In another aspect of the present invention, a system (e.g., apparatus)is provided which performs any of the surface polishing methods providedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding, reference should be made to the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1-1 is a schematic perspective drawing of a micro fast dischargeenvironment, in accordance with some implementations.

FIG. 1-2 is a schematic side-view drawing of a micro fast dischargeenvironment, in accordance with some implementations.

FIG. 2 is a functional block diagram of a micro fast discharge polishing(MFDP) system, in accordance with some implementations.

FIG. 3 is a functional block diagram of an MFDP station containing theMFDP system of FIG. 2, in accordance with some implementations.

FIG. 4 is a functional block diagram of a process of micro fastdischarge based surface polishing, in accordance with someimplementations.

FIG. 5 is a schematic of an MFDP chamber, in accordance with someimplementations.

FIG. 6-1 illustrates an active triggering mode, in accordance with someimplementations.

FIG. 6-2 illustrates a passive triggering mode, in accordance with someimplementations.

FIG. 6-3 illustrates a preionization mechanism to be used as controllingsignal for fast discharging, in accordance with some implementations.

FIG. 7 is a schematic diagram illustrating multiple MFDP systemintegrated into a single system with a common control center, inaccordance with some implementations.

FIG. 8 illustrates a process of using a plurality of MFDP systems inseries with different polishing specifications, in accordance with someimplementations.

FIG. 9 illustrates a one-dimensional “needle-type” electrode array, inaccordance with some implementations.

FIG. 10-1 illustrates an example of a scanning path of an electrodearray relative to an object that has a square or rectangular shape, inaccordance with some implementations.

FIG. 10-2 is illustrates an example of a scanning path of an electrodearray relative to an object that has a circular shape, in accordancewith some implementations.

FIG. 11 illustrates a layout of a scanning stage having calibrationareas, in accordance with some implementations.

FIG. 12 is a flow diagram illustrating a gas medium flowing loop, inaccordance with some implementations.

FIG. 13 illustrates an electrical energy reservoir that containsmultiple electrical charging and discharging sub-units, in accordancewith some implementations.

FIG. 14-1 illustrates a top view of a library of “needle-type”electrodes, in accordance with some implementations.

FIG. 14-2 illustrates a side view of a library of “needle-type”electrodes, in accordance with some implementations.

FIGS. 15-1 and 15-2 are flow diagrams illustrating a method of polishinga surface, in accordance with some implementations.

FIGS. 16-1 and 16-2 are flow diagrams illustrating a method of polishinga surface, in accordance with some implementations.

FIG. 17 is a process diagram illustrating a semiconductor process inwhich MFDP polishing is used, in accordance with some implementations.

Like reference numerals and names refer to corresponding partsthroughout the drawings.

DESCRIPTION OF IMPLEMENTATIONS

Reference will now be made in detail to various implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous specific details are set forthin order to provide a thorough understanding of the present disclosureand the described implementations herein. However, implementationsdescribed herein may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andmechanical apparatus have not been described in detail so as not tounnecessarily obscure aspects of the implementations.

The present invention described herein provides a system and method ofpolishing an object (herein referred to as “the object”) having asurface to be polished (herein referred to as “the surface”).

Modern semiconductor chips and integrated circuits are oftenmanufactured using photo-lithographic processes. One of the key stepsamong these processes is to transfer a circuit design on a photo-mask orreticle to a semiconductor wafer by using an optical imaging scannersystem. The production yield and quality of the resulting circuits relyon not just the precision of the optical scanner system but also thequality of the photo-mask and semiconductor wafer. For example, theuniformity and positioning accuracy of circuit features on thephoto-mask and the overlay accuracy of different wafer layers are someof the key factors that determine the final quality of semiconductorchips and the circuits thereon. Although many error sources caninfluence the accuracies described above, and contribute against a totalerror budget of modern wafer production, a few important error sourcesplay particularly determinate roles. Chief among these error sources aresurface flatness and roughness errors on both the photomask and thewafer, which cause problems in patterning overlay or uniformity.

With the evolution of technology and the increased demand for smaller,faster, and more powerful devices, the fields of semiconductormanufacturing and chip design are moving into the nanometer realm. Thatis to say, modern devices are characterized by a representativeline-width, sometimes called a space-width, on the order of nanometersor tens of nanometers. According to some predictions, the minimumfeature size of circuits on wafers will soon reach the 10-20 nanometerrange. This trend is expected to continue for a foreseeable time, sothat the total error budget of wafer production with high yieldrequirements will soon be greatly reduced. Furthermore, due to thelimited capability of conventional optical imaging systems, which arelimited by optical aberrations and the diffraction limit, thesemiconductor industry is expected to look towards unconventionallithographic systems (e.g., extreme ultra-violet (EUV) or soft x-raylithographic systems), in which a non-axial symmetrical imaging layoutis required. This imaging approach brings with it much tighterrequirement on optical aberrations, distortions and overlay errors, andtherefore implies strict requirements for surface accuracies such asflatness and roughness. These surface accuracy requirements will extendto such surfaces as the photomask surface (e.g., the photomask patternsurface), the imaging surface (wafer pattern surface and substratesurface) and surfaces of optical elements inside the lithographicsystem. For example, a surface flatness error (representing low andmiddle spatial frequency portions of a surface profile error) and asurface roughness error (representing middle and high frequency portionsof the surface profile error) contribute significant amounts to thetotal error budget, especially in terms of overlay error. Even smallamount of flatness error on the wafer surface can impact deviceperformance and process variation and therefore affect productionyields. Such error sources become more apparent and more severe when thewafer production moves into the realm of nanometer line-widths andbeyond.

As an example, some typical requirement data for future semiconductorproduction can be examined. According to some industry predictions, amaximum allowable flatness error (peak-to-valley) of 10-15 nm(nanometers) for photomask substrates and multi-layer blanks is expectedfor critical layers of the 16 nm line-width generation of semiconductorwafers. Root-mean-square (RMS) roughness error on photomask substratesfor the 16 nm line-width generation is expected to be on the nanometerscale. The requirements of surface flatness and roughness on waferblanks and processed wafers are severely restrictive as well. Theserequirements impose huge challenges for the future success of thesemiconductor industry.

Currently, surface polishing in the semiconductor industry is mainlyrealized by a chemical etching and mechanical polishing (CMP) method.Due to technical issues with this approach, surface polishing with CMPis expected to be limited to around surface flatness errors(peak-valley) on the order of micrometers or hundreds of nanometers.Because CMP is a global polishing approach (e.g., the entire wafer orchip, or at least a large portion thereof is polished at once), CMPpolishing tends to generate a signature “global bow” and also createsnew surface errors while attempting to minimize others. Other availablesurface polishing methods implemented in different fields (e.g., themechanical polishing, magneto-fluid, or ion beam bombardment methods),are tedious and time consuming, and thus cannot be easily transferredinto a high throughput production environment such as the semiconductorindustry. That is, to improve the throughput time of high accuracysurface polishing a new type of polishing technique is needed.

The present invention utilizes a fast electric discharge over a smallregion to destroy micro-scale surface undulations (sometimes referred toherein as “hills”). The discharging occurs between a sharp tip of a“needle-type” electrode and a surface of an object to be polished(herein, “the surface”), which have a strong electric field establishedbetween them. Due to the nature of the sharp shape of the tip of theelectrode, the electric field between the electrode and surface is verysensitive to variations of the surface undulations. A discharge circuitis used to generate very short discharging pulses on a timescale rangingfrom nanoseconds to micro-seconds, (sometimes called “fastdischarging”). Because the discharging occurs between the tip ofelectrode and the surface and only lasts a short period of time, thedischarging is localized both temporally and spatially, which results inhigh accuracy polishing. While the electrical discharging region isspecifically controlled by the electrode design and the circuit, thesurface to be polished can be continuously scanned using a stage, eithertranslational or rotational, so that the entire surface can be polishedwith production worthy throughput time. Since the surface polishingimprovement depends on the surface accuracy of the incoming surface, themethod is repeatable, meaning that better surface flatness and roughnessspecifications can be obtained by iterating the method repeatedly.

Additionally, the method described herein is valid for both flat surfacepolishing and curved surface polishing (e.g., spherical or other shapedsurface polishing). For polishing curved surfaces, an appropriateelectrode array is designed based on a model shape (e.g., a model of thesurface to be polished) and a scanning/rotating mechanism. Furthermore,the principle of the present approach can be extended to other fields,such as localized surface etching or surface repair, and can also beextended to use different types of pulses (e.g., other than electric)for surface polishing or figuring. The present invention is scalable tolarge-scale production and designed to be modular for compatibility withsupporting units, e.g., by changing the electrode array design.

An aspect of the present invention is to provide a novel high accuracysurface polishing system, for both flat surfaces and curved surfaces,which uses a micro-scale fast discharge approach with nanosecond pulseduration in a high pressure gas chamber (e.g., greater 1 atm(atmosphere)). In general, this system can be used for surface polishingcovering a wide range of materials and applications, said materialsincluding conductive materials, semi-conductive material, and insulatedmaterial coated with a conductive coating or liquid film. It is suitablefor high accuracy surface polishing because this novel method providespolishing capability within regions down to the nanometer scale, whichis controllable temporally in the range of nanosecond pulse duration.

Another aspect of the present invention includes a rotational ortranslational stage with high positioning accuracy for scanning theobject, which, in some implementations, is inside a high pressure gaschamber (sometimes called a “fast discharge chamber”) and is monitoredand controlled by an optical interferometer and/or an air-bearingsystem. The stage provides controlled movement in an x-y plane and highprecision leveling in a z-direction. The fast discharge chamber providesan environment for surface polishing which is capable of supporting botha vacuum condition as well a condition in which the chamber is filledwith a discharging medium to a high pressure with a gas pressure valueranging from a few atmospheres (ATM) to tens of atmospheres (ATM). Thegas pressure value is determined in accordance with the requirements ofthe discharging pulse duration as well as the breakdown voltage of thedischarging medium. In general, shorter pulse durations require higherpressure and the help of a quick switching mechanism (described below).Besides the stage, the system includes one or more electrodes, which arecollectively referred to as an “electrode array” (i.e., an electrodearray is one or more electrodes). The system also includes a dischargingmedium flowing and cycling sub-systems for sustaining uniform dischargecharacteristics and extending the lifetime of the discharging medium, afast discharge electrical circuitry sub-system, and a dischargemonitoring and controlling sub-system. The apparatus can operate in an“active” fast discharge mode or “passive” fast discharge mode, and eachmode can have differing arrangements to support the generation of veryshort electrical discharging pulses.

One advantage of the present invention is that its unique fast dischargeapproach can contain polishing to within a discharging region of thefast discharge, and therefore the surface polishing is localized bothtemporarily and spatially. In principle, the discharge occurs between atip of an electrode (e.g., within an electrode array) and a surfacehill. The strength of the electric discharge is extremely sensitive tothe height of the surface due to the steep variation in the electricfield strength, and is therefore localized to the hill. The dischargeprocess only lasts for a period of a few nanoseconds or tens ofnanoseconds, which can effectively prevent the extension of thedischarge into neighboring regions. The realization of localized fastdischarging, both temporarily and spatially, can be controlled andoptimized by the geometrical design of the electrode, the pulse-width ofdischarging pulse, the triggering mechanism of the discharging, thepressure and mixture of discharging medium, and others. Therefore, thesefactors provide a wide range of processing recipes and an ability totune the system to offers extensions to a variety of applications.

Another advantage of the present invention is the ability to modify thebreakdown voltage of the discharging medium by tuning the gas pressurevalue of the discharging medium. Thus, one can modify the amount ofelectrical energy injected into the surface hills so that the injectingintensity generated by discharging process can be moderated. Byadjusting the gas pressure value of the discharging medium and theswitching time of a triggering switch, the breakdown voltage and pulseduration can be tuned and optimized for different materials andpurposes. Based on input data of the surface flatness and roughness, thedischarging distance between electrode and the surface undulations canbe calibrated using a reference plane. Due to the sensitive nature ofelectrical discharging around a sharp tip of a “needle-type” electrode,a few percent variation of the surface height can cause tens of percentor even larger variation of the electric field strength between theelectrode and the surface hill, which can trigger the dischargingprocess automatically. A steep rising edge of a short discharging pulsefurther enhances this process.

In yet another aspect of the present invention, a fast discharginglayout is used in concert with a 1-dimensional electrode array or a2-dimensional electrode array to improve the polishing efficiency andproduction throughput, which is important for high volume production.The electrode array uses either equally-distanced electrodes or anasymmetrical arrangement of electrodes. In some implementations, theasymmetrical arrangement of the electrodes is characterized byseparations between the neighboring electrodes that are differing primenumbers of unit distances apart (e.g., 1 unit distance, 3 unitdistances, 5 unit distances, 7 unit distances, 11 unit distances, etc.,where the unit distances can be measured in any suitable unit, such asnanometers, microns, mils, etc.) so that the scanning paths from thedifferent electrodes will not overlap while the stage is scanning ineither a rotational scanning mode, a translational scanning mode, orcombination thereof. As shown in FIG. 9, each electrode within theelectrode array is independently controlled for charging, triggering anddischarging by its own electrical circuit connected to an electricalenergy storage reservoir.

The triggering mechanism of the micro fast discharging can be either anactive discharging mechanism (or mode) or a passive mechanism (or mode).In the active mode, the system receives surface undulation data as aninput from, for example, a surface metrology system (e.g., flatnessmetrology system). The system then converts the input data into pixeldata matching a scanning path of the scanning stage. The pixel sizedetermines the minimum polishing region and the fineness of thepolishing process. The pixel size is determined by a convolution of theminimum pixel size of the metrology data and the electrode design. Thescanning path can optionally be from pixel-to-pixel or from one unit tothe next unit that contains a pixel value. For ultra-fine polishing, ifa single pixel of the metrology data is large compared to a localizedregion of surface error, the scanning unit or discharging unit can besplit by scanning path design. Using the metrology data, the dischargingcondition at each pixel is determined and the discharging is triggeredby a triggering signal (e.g., an electrical and/or optical signal).

In some circumstances, the passive mode is used when either metrologydata is not available or it is not beneficial. In such circumstances,the discharging can be self-triggered based on a pre-defined thresholdcondition (e.g., a threshold distance between the electrode tip and asurface hill). Along a scanning path, a discharge will occur at alocation when a surface parameter breaks the threshold. In someimplementations, the pre-defined threshold condition represents amaximum acceptable level of surface topography variation, and it isdetermined by surface polishing specifications of the outgoing objectsunder incoming material condition. The comparison between thepre-defined threshold condition and the surface parameter is performedat each pixel. In some implementations, the surface parameter (e.g. thedistance between the tip and the surface) is measured in real-time usingvarious methods (e.g., an optical height sensor or by measuring avariation of the electric field strength around the tip of theelectrode). Due to the geometrical structure of the electrode, thevariation of the electric field strength around the tip of the electrodeis sensitive to the variation of surface flatness and roughness andtherefore has a great capability for ultrafine surface polishing.

Another aspect of the present invention is the flowing and cycling ofthe discharging medium (gas mixture) within the discharge chamber, sothat heating caused by the discharge can be dissipated, the dischargingmedium lifetime can be extended, and the discharging quality can beimproved. The process of the discharging medium flowing and cycling alsocleans debris from the surface while the discharging occurs. Thedetermination of the gas flow rate is determined in accordance with thedischarge repetition rate. In each cycle, a small amount of freshdischarging medium is injected into a flowing channel of chamber and anequal amount of used discharging medium is drained out and stored in aprocessing storage cylinder for filtering and reuse. Real-timemonitoring and filtering of the discharging medium is used forcontinuous high volume production.

Considering the high accuracy of the surface before and duringpolishing, in some implementations, the electrode and/or electrode arrayare calibrated before and/or during the polishing process. Someimplementations of the stage include features that provide measurementand calibration capability for surface leveling and dynamic calibrationand micro-adjustment of the electrode array. In some implementations, aplurality of predetermined points on the surface are monitored andcorresponding z-coordinate is measured for each point using an opticalinterferometer or height sensor before and/or during the stage scanningmovement. The z-coordinate (e.g., height) of these points can be used todetermine, for example, one or more of a pitch, a roll, and a yaw, aswell as an overall height offset of the surface with respect to areference surface (e.g., fiducial surface). These quantities can then beused for calibration and adjustment in a real-time mode. In someimplementations, along the sides of the stage, (however, separate fromthe polishing object), there are one or more calibration panels thathave very high surface accuracy, which serve as reference region forelectrode array calibration. The electrode array calibration isoptionally performed before the polishing and after each cycle of stagescanning For this reason, the electrode array is mounted on an electrodehead that is capable of performing adjustment in the z-direction, e.g.by using an ultra-fine screw unit.

Due to the very strong electric field strength surrounding the tip ofthe electrode during discharging, the shape of electrode tip issometimes eroded or otherwise changed and therefore the polishingquality can be degraded. To improve the quality and efficiency of thepolishing process, a library of electrodes (e.g., a cassette or magazineof electrodes) is provided in the present invention, which provides away to quickly exchange electrodes when the degradation of an in-useelectrode drops below a pre-defined criteria (e.g., a pre-definedcriteria set by a user). After the electrode exchange, a new electrodeis calibrated using the calibration panels. The electrode library isespecially valuable when a large electrode array is utilized, which, asmentioned above, is used to greatly increase the throughput of polishingprocess.

As another aspect of the present invention, multiple micro fastdischarge polishing (MFDP) systems can be arranged in series so a firstsystem's outgoing object is the incoming object of a second system.Likewise, the second system's outgoing object becomes a third system'sincoming object, and so on. In this way, each system only needs topolish a limited and pre-defined range of surface errors and thesystem's calibration and operational tuning for better stability issimplified. In addition, one or more of the MFDP systems can havemultiple discharge chambers with single central control unit, which cangreatly enhance the efficiency and throughput of the polishing process,especially for those objects that have similar surface qualities (e.g.,similar peak-valley flatness ranges) and the same polishingrequirements. Because the system experiences vacuum conditions andundergoes filling of the discharging medium during object loading andunloading, to reduce the cycle time of the polishing process and enhancethe system productivity, a library for object storage (e.g., a cassetteor magazine to hold wafers) is, in some implementations, attached to thedischarge chamber. The library for object storage is operated under thesame gas pressure conditions as the discharge chamber (albeit,optionally with no gas flow). In this way, the discharge chamber canmaintain its operational conditions until all of the objects in thelibrary are polished.

To achieve and maintain high throughput in a manufacturing environment,some implementations utilize high speed scanning and a high dischargingrepetition rate. Therefore, in some implementations, the system includescorresponding high speed electrical charging and discharging systems.Circuitry for a single fast discharge (e.g., without recharge) andapplications thereof are well-known in the art and have been utilized indifferent fields. However, in some circumstances, a singlecharging/discharging unit is not sufficient to meet the requirements ofhigh scanning speed, decreased size of the polishing pixel andimplementation of multiple electrodes. To meet these requirements, someimplementations provide an energy reservoir that is attached to the MFDPsystem, which is designed to realize the high speed electrical energycharging and discharging. In some implementations, the reservoircontains an array of energy storage units that have cylindricalsymmetric layout, although other layout types are possible as well, andall of units are independent of each other. There are two electricalsub-systems within the reservoir: a charging sub-system and adischarging sub-system. These sub-systems connect to the reservoirindependently. In the discharge processing, each individual electrode ofthe electrode array is connected to a respective energy reservoir unitin an exchangeable way. When the system detects that an energy level ofthe respective energy reservoir unit drops below a pre-defined limit,the electrode connected to the respective energy reservoir unit isswitched to a new energy reservoir unit, while the exhausted energyreservoir unit is then coupled to the charging process.

To avoid interference between the MFDP system and other processequipment surrounding it, some implementations of the MFDP systemprovide an environmental control enclosure that includes multipleenvironmental control capabilities including electromagnetic (EM) fieldshielding, tight vibration isolation, acoustic noise isolation andtemperature variation control. A discharging medium purification forparticle filtering is also included in the environmental controlenclosure.

Overall, the MFDP system provides a novel method and technical approachfor high accuracy surface treatment. Although many parameters and systemfunctions contribute to this new capability, a few key parametersdetermine the fundamental achievable capability. These include a minimumsize of the electrode tip, a minimum distance between the electrode tipand the surface and an ability of the scanning mechanism (e.g., thescanning stage) to accurately control position, as well as the minimumachievable electrical temporal pulse-width of the discharging pulse. Asan example, an electrode tip with a characteristic size on the order oftens of micrometers or a few micrometers, with a temporal pulse-width ofthe discharging pulse on the order of hundreds of nanoseconds, issuitable for micrometer or sub-micrometer surface polishing processes.As another example, when using an electrode with a characteristic sizeof a few nanometers (e.g., up to tens of nanometers) with a temporalpulse-width of the discharging pulse on the order of a few nanosecond(e.g., as measured by the peak-valley flatness error or low-mediumspatial frequency surface error), nanometer level polishing isachievable.

As currently understood, λ/100 surface polishing accuracy is theso-called classical limit of conventional technologies, where λ is awavelength of a characteristic light being used (e.g., a wavelength ofhelium neon (HeNe) laser radiation, which is 632.8 nm). This means thatthe current classical limit is about 6-7 nm peak-valley surface accuracywithin a surface area of few hundred square centimeters. Moreover, thisis achievable only in a slow and time consuming way and therefore onlysuitable for small quantity production. The MFDP system and technologydescribed herein overcomes this limitation because of its spatially andtemporally localized nature, allowing for point-to-point surfacepolishing using a discharging pulse generated by breakdown of adischarging medium subject to an electric field. This is used inconjunction with high speed object scanning for high volumemanufacturing.

Reference will now be made in detail to various implementations,examples of which are illustrated in the accompanying drawings. In thefollowing detailed description, numerous specific details are set forthin order to provide a thorough understanding of the present disclosureand the described implementations herein. However, implementationsdescribed herein may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andmechanical apparatus have not been described in detail so as not tounnecessarily obscure aspects of the implementations.

Operational Theory

Now referring to the drawings, FIGS. 1-1 and 1-2 illustrate theoperational theory of a micro fast discharge polishing (MFDP) system, inaccordance with some implementations. An electrode 100 scans a surface102 of an object 103 to be polished. A region of space between theelectrode 100 and the surface 102 defines a discharge cavity 104. Adischarging medium (e.g., a gas mixture) is introduced into thedischarge cavity, and a voltage is applied between the electrode 100 andthe surface 102 resulting in an electric field within the dischargecavity 104 characterized by an electric field strength, E. A dischargingpulse occurs when a variation in the distance between the electrode 100and the surface 102 or a variation in the voltage applied between theelectrode 100 and the surface 102 results in electric breakdown of thedischarging medium. In some implementations, the electrode 100 does notmove within the x-y plane and only performs height adjustments alongz-direction, while the scanning stage that holds the object 103 performsthe scanning movement in both the x and y directions.

In some implementations, the electrode 100 is a “needle-type” electrodemeaning that the electrode 100 has a sharp tip that is proximal to thesurface. Due to the sharp tip of the electrode 100, the electric fieldstrength surrounding the electrode tip is very sensitive to variationsof the surface's 102 topography. The electric field strength near thetip of the electrode 100 is proportional to r^((π/β−1 )), where r is thedistance between the tip and a field point 106, and β is an outer angleof the tip. When β approaches 2π and r approaches 0, the electric fieldstrength is extremely large and sensitive to variations of r. That is tosay, a small amount of variation in the distance between the electrode100 and the surface significantly changes the electric field strength.This effect becomes more severe if the electrode tip is very close tothe surface 102. Therefore, this geometrical structure has greatpotential for high accuracy surface treatment, and the resulting surfacetreatment is limited primarily by the size of the electrode tip and theminimum distance between the electrode tip and the surface 102 that canbe achieved. In some implementations, a reference surface for polishingis correlated to a threshold distance 108 that will trigger discharging.Where the gap between the surface 102 and electrode 100 is narrower thanthe threshold distance 108, the electric field strength generated by afast discharging pulse becomes stronger and therefore the strengthdelivered by the discharging pulse is stronger on the hills 110 (e.g.,hill 110-a and hill 110-b) of the surface 102 than in the valleys 112.This strength increase is especially sensitive both because of theelectric field created by the applied voltage and because of the natureof breakdown discharge from an electrode tip, which is sensitive to thedistance change and field strength increase in a non-linear fashion. Thepresent technique self-adjusts the polishing strength in accordance withthe surface error height. In other words, a higher hill 110 (e.g., withrespect to the electrode tip) will receive more polishing, while a lowerhill 110 will receive less polishing compared to the former case,provided that the electrode tip maintains an unchanged height in thez-direction. Because the discharging pulse is confined between theelectrode tip and the hill 110, this polishing approach is sometimesreferred to herein as “point-to-point” surface polishing. In nature, ithas huge potential to “crash” hills 110 and achieve very high surfacepolishing accuracy.

Inside the polishing chamber, a discharging medium (e.g., a highpressure gas mixture) is filled and flowed continuously. Dischargingpulses are generated via breakdown of the discharging medium. Thedischarging pulses are characterized by a pulse-width that depends onthe characteristics of the discharging medium (e.g., composition,pressure, etc.), the characteristics of the discharging cavity (e.g.,electrode shape, distance between the electrode and the surface, etc.),as well as characteristics of a fast discharge circuit described below.In this way, discharging pulses can be generated with short pulse-widths(e.g., from a few nanoseconds to tens nanoseconds). During this time,electrical energy stored in the fast discharge circuit is released intoa very small spatial region restricted by the sharp tip of the“needle-type” electrode. Thus the discharge process is well containedaround non-flat spots on the surface, both in time domain and spatialdomain, so that over-polishing and/or damage of neighboring areas isavoided.

There are multiple system parameters that a user can tune for differentapplications and specifications (e.g., flatness requirements). Forexample, the pressure and composition of the discharging medium, thesize and shape of the electrode, the composition of the electrode, themechanism of discharge triggering and gating, the energy release rateand the pulse duration can all be tuned or otherwise adjusted. Becauseof this flexibility, this novel technology provides a broad range ofcapabilities that are suitable for different applications in variousfields. To name a few, the systems, methods, and processes describedherein provide ultra-fine surface polishing or treatment on wafers andphotomask substrates as well as patterned surfaces used in thesemiconductor industry. In addition, the system, methods, and processesdescribed herein can provide surface polishing in optical componentmanufacturing, or in other fields that need highly precise surfaces. Thesystems, methods, and processes are capable of polishing variousmaterials (e.g., conductors, or semiconductors, or insulating materialcoated with a conductive coating). By using various molds and tooling aswell as different scanning mechanisms, this surface polishing capabilitycan be extended from flat surfaces to curved surfaces. For example, forspherical surface polishing, the object can be scanned or rotated arounda spherical center, while an electrode array can be mounted on anotherspherical surface that has common spherical center with the object.

Operational Structure and Layout

FIG. 2 shows a block diagram of a micro fast discharge polishing (MFDP)system 200 of the present invention. System 200 includes a main station202 that performs the polishing operation and several supportingsub-systems, including: (1) a vacuum sub-system 204 for guaranteeing thepurity and proportion of the discharging medium (e.g., after objectloading and/or unloading); (2) a discharging medium filling and cyclingsub-system 206; (3) an energy storage sub-system 208 for electriccharging and discharging support; (4) a cleanliness and environmentcontrol sub-system 210; (5) a post-processing sub-system 212, fordischarge debris clean-up and residue charge removal; (6) an objectloading/unloading/transferring sub-system 214 optionally includingloaders, clean room robots, and so on; (7) a computer sub-system 216 forsoftware and automation; and (8) a power, utility and electrical controlsub-system 218. The structure and layout of present invention aredesigned for practical integration into high volume manufacturingenvironments.

FIG. 3 shows the functional block diagram of the main station 202. Thecentral part of the main station is a high pressure gas fast dischargechamber 302. Other optional sub-units include: (1) a fast dischargecontrol sub-unit 304 for discharge timing/cut-off control, anddetermination of threshold conditions; (2) an object alignment,leveling, and reference plane calibration sub-unit 306 for aligning,leveling, and determining a reference plane of the surface of theobject; (3) a real-time monitoring, compensation and logging sub-unit308 for automation of, for example, polishing, scanning, etc.; (4) arotational and/or translational stage unit 310 for controlling movementof the object and performing tasks such as calibration andself-adjustment; (5) a data input/output (IO) unit 312 for retrieving,storing and handling data (e.g., metrology data, discharge currents andtimes, etc); (6) a computer algorithm, station control and softwaresub-unit 314; and (7) a electrode (array) calibration sub-unit 316.

FIG. 4 is a flow chart of a polishing method 400. In someimplementations, polishing method 400 is implemented on the system 200.Because the incoming and outgoing objects on the system 200 generallyhave high accuracy surfaces and are, in some circumstances, held toextremely high cleanliness requirements, in some implementations, all ofthe operations of the method 400 are contactless (e.g., do not touch thesurface) and are handled by a robot.

Method 400 includes transferring (402) one or more objects to a loadinglibrary of the system. In some implementations (e.g., for use a highvolume production environment), multiple objects are stored in a singlecassette or magazine and transferred into the loading library at thesame time (e.g., the system loads them in the loading (storage) libraryand processes them serially or in parallel).

Because of the need to load and unload the objects (e.g., wafers), thedischarge chamber 302 cycles between dry air and the discharging medium.To this end, the method 400 includes regulating (e.g., venting)(404) thedischarge chamber 302 to normal (e.g., atmospheric) pressure. In someimplementations, the discharge chamber is vented under a positivepressure of an inert gas such as nitrogen.

Method 400 further includes opening (406) the chamber door and loadingone or more objects (e.g., those objects in the loading library) onto astage (e.g., onto a chuck that mounts the object to the stage). Method400 further includes closing (408) the chamber door, and performing aninitial alignment of the stage at normal (e.g. atmospheric) pressure. Insome implementations, the initial alignment is a gross alignment. Insome implementations, the electrode array undergoes an initialcalibration.

The method 400 further includes vacuum pumping (410) of the chamber(e.g., for cleanliness). A stage alignment is performed under vacuumpressure and the electrode array is calibrated for a second time.

The method 400 further includes starting (412) gas flow of thedischarging medium. In this manner, the chamber is filled with thedischarging medium to a working pressure.

The method 400 further includes performing (414) a final stage alignment(e.g., a fine alignment), initiating dynamic correction, and performinga third electrode array calibration.

The method 400 further includes loading (416) software (e.g., controls,user interface software, etc.) and data (e.g., metrology data in theactive mode) for polishing. A parts, timing and origin set check is alsoperformed, in accordance with some implementations.

The method 400 further includes scanning (418) the stage to performpolishing with dynamic correction of coordinates, leveling, electrodearray position and status. In some implementations, the dynamiccorrection of coordinates is a real-time correction that is performed inthe x-y plane as well as in the z-direction. In some implementations,the dynamic correction is performed after a particular path is scanned(e.g., at the end of a “line-scan” across the object). In someimplementations, while performing the dynamic calibration and alignmentafter the scanning path, the position of electrode tip is measured andadjusted. If necessary, an electrode in the electrode array is swappedand returned to the correct position relative to the object.

After scanning the stage, gas flow is continued (420) for cleaning.After a predetermined condition is met (e.g., after a predeterminedamount of time that the gas flow is continued) the chamber is pumped tovacuum levels.

Finally, the method 400 includes unloading (422) the object to a carrier(e.g., a track that will move the object to a wafer box, magazine, orcassette for unloading).

In some circumstances, the discharge chamber 302 is vacuumed before andafter each gas switch (e.g., operation 410 is repeated). However, insome circumstances, the vacuum level of the discharge chamber 302 is notvery high for the object loading/unloading operations. To save cycletime and improve throughput, in some circumstances it is beneficial toinclude an internal storage library that can contain multiple object(e.g. a wafer lot box, cassette, or magazine). When the storage unit hasthe same gas conditions as the discharge chamber 302, the polishingprocess will continue without the need to switch between the dischargingmedium and a vented condition until the all of the objects in theloading library are processed. The resultant quality of the polishingprocess relies upon several factors: the quality of the calibration ofelectrode array, the object alignment and the ability to perform thereal-time correction of the relative position between the electrodearray and features (e.g., “hills”) of the surface.

The details provided with reference to the method 400 are only intendedto provide an example of a method for polishing a surface and are notintended to limit the claims that follow. For example, the requirementsand procedures of calibration and alignment may vary.

Main Discharge Chamber

FIG. 5 shows a schematic of the discharge chamber 302, in accordancewith some implementations. The discharge chamber 302 provides anenvironment for high accuracy surface polishing and isolates thedischarging process so that the process inside the discharge chamber 302and activity outside the discharge chamber 302 do not interfere witheach other. The discharge chamber 302 includes multiple sub-units,including: (1) a scanning stage 500 that holds the object 103 (e.g.,with a vacuum chuck) and moves the object 103 in accordance withuser-defined procedures, (2) an electrode head 502 that contains anelectrode array and movement controls, (3) a library of electrode arraysfor electrode replacement, (4) a gas flow control sub-unit, (5) amonitoring sub-unit for process checking and tuning, and others. In someimplementations, the stage is electrically grounded and the electrodehead is held at a negative voltage.

In some embodiments, the stage 500 also includes one or more calibrationpanels 504 (e.g., calibration panel 504-a and calibration panel 504-b),which are described in greater detail below.

In some embodiments, the discharge chamber 302 includes one or more gasinlet ports 506 (e.g., gas inlet port 506-a, gas inlet port 506-b, andgas inlet port 506-c). In some embodiments, the discharge chamber 302includes one or more gas outlet ports 508 (e.g., gas outlet port 508-a,gas outlet port 508-b, and gas outlet port 508-c). The gas inlet ports506 may be coupled with one or more gas source lines (e.g., eachproviding a different gas), while the gas outlet ports 508 may becoupled with an exhaust system. The flow of gas through the gas inletports 506 and the gas outlet ports 508 serves to introduce and removegases (the discharging medium, air, nitrogen, and the like) to and fromthe discharge chamber 302. Flow of gases through the gas inlet ports 506and the gas outlet ports 508 is controlled, in some embodiments, by thegas flow control sub-unit.

The discharge chamber 302 is constructed in accordance with variousmechanical, electromagnetic (EM) and environmental control requirements.To obtain a high quality processing discharging medium and achieve highquality discharging, the discharge chamber 302 supports a vacuumenvironment which is typically used before gas filling (see operations410 and 412, method 400, FIG. 4). The discharge chamber 302 furthersupports a high pressure environment, in which the discharge chamber 302is filled with the discharging medium to a high pressure, for use duringthe polishing processing, and also supports a normal pressure (e.g., 1atmosphere) dry air (e.g., nitrogen gas) environment for object loadingand unloading. A typical gas pressure range for nanosecond fastdischarging is in the range of a few atmospheres to tens of atmospheres.In general, shorter discharging pulses utilize a higher pressure of thedischarging medium. Therefore, the discharge chamber 302 is designedwith mechanical strength and gas sealing specifications that depend uponthe designed discharging pulse time. Due to the fact that the MFDPprocess utilizes repetitive electrical charging and discharging, thedischarge chamber 302 provides good electrical performance andinsulation, and, in some implementations, also provides electromagneticshielding and body grounding.

The discharging process inside the discharge chamber 302 can generatestrong acoustic noise. Therefore, in some implementations, acousticnoise reduction and shielding are provided.

Because the discharging process will generate heat, even with gas flow,some implementations of the discharge chamber's 302 design include heatexhaust, cooling and temperature control. These elements of thedischarge chamber 302 maintain a stable and reliable environmentcompatible with polishing processes for high-accuracy surface flatnessspecifications. For example, these elements of the discharge chamber 302provide an environment in which error sources caused by processdeviations, including fluctuations of the gas flow rate, the electricalvoltage between the object and electrode array, stage positioningaccuracy, temperature and so on, result in errors amounting to a smallportion of the surface accuracy and surface flatness specification(e.g., 10%).

Modes of Fast Discharging

Two polishing modes (e.g., modes of controlling polishing) areprovided: 1) an active mode of micro fast discharge triggered that iscontrolled (e.g., triggered) by flatness and/or roughness data resultingfrom measurements of the surface (e.g., measurements performed prior tothe start of polishing using a surface metrology measurement system),and 2) a passive mode (also called self-consistent mode or aself-adapted mode) of micro fast discharge that is triggered by thevariation of surface profile. In either case, a preionization signal canbe used to facility the trigger of a discharge pulse.

Active Mode:

FIG. 6-1 illustrates the active mode (sometimes called controlled mode)of micro fast discharge polishing (A-MFDP). The fast discharge in thismode is controlled by either flatness metrology data for low to mediumspatial frequency surface errors and/or roughness metrology measurementdata (e.g. atomic force microscopy data or scanning electron microscopydata), for medium to high spatial frequency surface errors. When theelectrode 100 is over a region of the surface 102 that has acorresponding data pixel that exceeds a threshold value, the systemsends out a signal to initiate fast discharge and also determines thestrength of the discharging electric field using a magnitude of thesurface error (e.g. peak-to-valley magnitude). For example, in someimplementations, an amount by which a data value exceeds the thresholdvalue is used to determine the strength of the discharging electricfield. To this end, a measurement data file is used as an input file andconverted into pixel data based on a scanning mechanism (e.g., linearscanning, circular scanning and/or localized scanning). Localizedscanning is particularly useful when there are a number of small regionsthat need polishing. The threshold value of discharging is dependent onvarious parameters including a user's specification, an incoming surfacecondition, the discharging medium composition and pressure, and so on.The electrode 100 and the surface 102 to be polished are charged by afast discharge electrical circuit, in which the electrical voltagebetween the electrode 100 and the surface 102 is, in someimplementations, slightly below the threshold condition (option 1).Alternatively, the electrode and the surface are held at an equalvoltage before discharging (option 2). In some implementations, thesurface is grounded and a negative electrical voltage is applied to theelectrode array.

In option 1, when an imported input pixel shows that a surface parameterat a forthcoming point is above a pre-defined threshold value, a triggersignal 600 is sent out to trigger a pre-ionized weak discharge, whichwill trigger a main discharge. In this option, the fast discharge iscontrolled using a switch 602 (e.g., a gas-filled tube or other switchwith fast response time). For example, the default position of theswitch 602 is an “on” position prior to discharge, but is turned offwhen the surface discharge is to be ended for pulse duration control.Before a second signal based on the metrology data arrives, the switch602 is returned to the “on” position, the discharging loop is rechargedand the readiness of discharge circuit is restored.

In option 2, the electrode 100 and the surface 102 are held at an equalvoltage prior to discharge and a very short high voltage pulse isapplied to the discharge loop whenever the switch 602 is turned on,which uses the data from the input file that contains the surfacemetrology data. Because the discharge cavity is filled to a highpressure with the discharging medium (e.g., helium, or nitrogen, and/ormixture thereof), a fast discharge that is on the order of nanosecondcan be achieved. The discharge is a result of breakdown of thedischarging medium that can be understood using transmission linetheory: when the switch 602 is suddenly turned on, the electromagnetic(EM) field wave is transmitted to the electrode 100. Due to reflectionof the EM field wave at the electrode, the electric field between theelectrode 100 and the surface 102 is enhanced (e.g., the electric fieldis doubled or otherwise significantly increased) and therefore forms asharper discharging pulse. For the active mode, in terms of triggeringmechanism, option 2 has an advantage that it is easily implemented. Thisis sometimes called “Blumlein” type fast discharge.

To reach higher accuracy polishing, this polishing procedure can berepeated. Thus, a result of a current polishing process can serve as apre-treatment for a subsequent polishing process. However, in someimplementations, the discharge conditions and triggering threshold aremodified for the subsequent polishing process. In some implementations,the subsequent polishing process is performed in a separate chamber.Alternately, the subsequent polishing process is performed in the samechamber using, for example, a different recipe. In some circumstances,because the nanosecond fast discharge is very powerful, a damagethreshold of the surface is determined as part of a process ofdevelopment and commissioning that can be used to identify the properdischarging parameters. As stated, in both options, in the repetitivedischarging process, charging, discharging and switching speeds aredesigned to be high enough to match the speed of scanning and the pixeldensity. To this end, the present invention provides an energy reservoirfor this purpose, which is described in following section.

Passive Mode:

As shown in FIG. 6-2, passive mode (self-consistent mode) micro fastdischarge polishing (P-MFDP) is controlled by variations of the surfacetopography error (e.g., either non-flatness error or roughness error).Passive micro-fast discharging polishing is sometimes referred to hereinas “passive and adaptive fast discharge.” The physical principle of thisapproach is to use the large electric fields around the tip of everyhill 110 (e.g., hill 110-c) or sharp surface topography variation, inwhich the voltage for the discharge is applied between the electrode 100and the surface 102, to self trigger the discharging pulse. According todischarge physics, a variation of a few percent in the distance betweenelectrode 100 and the surface 102 can cause large electric fieldvariation, ranging from tens of percent increase to double the originalvalue. When the voltage between the electrode 100 and the surface 102 isset slightly below the breakdown voltage, this strong electric fieldvariation will suddenly trigger the discharge. Because the electrode tipcan be made as small as hundreds of nanometers and the dischargedistance can be controlled at the sub-micrometer level, this approachcan perform high accuracy polishing, ranging from the micrometer to thenanometer level, depending on the original surface flatness androughness level and the quality of the electrode 100 and/or electrodearray as well as the accuracy the stage leveling and control of thestage movement.

In some implementations, the electrode 100 and the surface 102 arecharged to their relative voltage by a capacitor circuit with lowelectrical inductance. In some implementations, the circuit includes adischarge termination switch (analogous to the switch 602 in FIG. 6-1)that is in a default “on” position when no discharge occurs. Thedischarge termination switch is turned off very quickly when dischargeoccurs and a feedback signal from a sensor (detector) in the dischargeloop is sent back to this switch to terminate the discharge, which isthe option 1 triggering mechanism stated in previous section. In someimplementations, the switch is a “discharge type” switch that fills witha high pressure gas (helium or nitrogen and/or a mixture thereof) toincrease the switching speed. At a few atmosphere pressures, theswitching speed can be controlled at the nanosecond level so that veryshort pulse discharges can be realized. After discharge is terminated,the switch is placed back into the default “on” position and the loopbegins the recharging process. This procedure will occur again wheneverthe electrode 100 encounters a sharp hill 110 in the surface topographyduring the scanning, which triggers another fast discharge event. Thescanning path in this scheme can be one of several options (e.g.,circular scanning, x-y scanning and/or localized scanning), depending onthe dimensional shape of object and condition of surface errordistribution. The passive mode can also utilize the option 2 triggeringmechanism, in which both sides of the discharge cavity are held at thesame voltage prior to discharge and a default “off” switch turns on tobegin the discharge process. When the discharge ends, the switch returnsto the “off” position. In the option 2 configuration, the triggeringsignal is from the variation of surface topography, which can bemeasured and given by for example, a height sensor 604.

Material and Conductivity

The effectiveness of the fast discharge polishing process is increasedif the material to be polished has a high electrical conductivity. Forsemiconductors or low conductivity materials such as insulators, thefast discharge situation is different compared to fast discharge usingconductive materials, and therefore different process parameters areused. Namely, the difference of material conductivity will dictate adifferent discharging threshold and pulse profile, and therefore othertechnical methods may be implemented to improve the dischargingeffectiveness and smoothness. For example, different process recipes aredefined with different discharge parameters for different materials. Inaddition (or alternatively), a coating of a conductive film (e.g., witha thickness of a few nanometers to tens of nanometers), can be appliedto the surface 102 to increase the effective conductivity for thepolishing process and then removed (e.g., by sputtering) after polishingprocess ends. As another example, a thin film of purified water, withappropriately conductive additions, spin coated on the surface can beused and subsequently cleaned and/or evaporated after the polishingprocess. When the surface 102 has a complex pattern or mixture ofdifferent materials (e.g., a patterned semiconductor wafer surface),pattern data and material parameters including conductivity distributioncan be integrated into a data preparation file in the active mode andthe discharge conditions can be pre-set accordingly (e.g., timing ofpreionization, discharging pulse duration and discharge cut-off time,and so on). Alternately, or in addition to, the process can besimplified by use of an assisting coating applied on surface 102.

Scanning Mechanisms

There are multiple options for relative scanning mechanism between anelectrode 100 or electrode array (1-dimensional or 2-dimensional) andobject 103 mounted on the stage. In some implementations, the electrodearray only moves in the z-direction and the object 103 is mounted on astage that provides x-y movement (e.g., either translational orrotational or localized movement).

FIG. 9 illustrates multiple “needle-type” electrodes comprising a1-dimensional (1-D) array. Each “needle-type” electrode 100 in the array(e.g., electrodes 100-a through 100-n) can be adjusted independently soas to avoid contact with the surface due to the surface flatnessvariations. The 1-D electrode array can be designed in multiple ways.For example, (1) the distance (or ratio of the distance) betweenneighboring electrodes 100 is one of prime numbers or non-integernumbers so that scanning paths will be interlaced but not overlapping,or (2) the distance between neighboring electrodes 100 is a constantfirst distance, where the constant first distance is smaller than theminimum pixel size provided by surface metrology data (to increase adischarge striking density to meet polishing requirements, which may beuseful if the dimensional size of object is big and metrology data pixelsize is also big), or 3) the distance between neighboring electrodes 100is a predefined second constant distance, where the minimum pixel sizeis smaller than the predefined second constant distance. The latterarrangement is used in conjunction with a stepping movement after eachscanning path that steps a fraction of the distance between electrodeswithin the electrode array, so that the entire region surface iscovered.

These options are provided only as examples to aide in a more thoroughunderstanding of the present invention, and are not intended to limitthe claims that follow. Moreover, any combination of scanningmechanisms, stepping movements and electrode array arrangements thatminimizes multiple fast discharging events at same location and missedspot may be used, as well as others.

Electrodes 100 can also be arranged in 2-D array to increase throughput(e.g., reduce the time) of the polishing process. In a 2-D array,different rows of electrodes 100 can be shifted in position relative toa neighboring row (e.g., assuming all of rows have the same distancebetween electrodes). For example, in a three row array (having a firstrow, a second row, and a third row, each with rows of electrodes 100placed along a row direction), the second row of electrodes 100 can beshifted relative to the first row along the row direction one third ofthe distance between neighboring electrodes 100, and the third row canbe shifted along the row direction an additional one third of thedistance relative to second row. When the minimum pixel size of themetrology data is much smaller than distance between electrodes 100,this approach will cover more locations on the surface in each cycle andeffectively reduce the polishing time. In a situation in which theminimum pixel size is large compared with the distance between twoelectrodes 100, this approach can also reduce the effective polishingarea and increase the overall quality of surface polishing.

Movement of the Object

As shown in FIG. 10-2, in some implementations, an object 1002 rotatesaround a central axis (parallel to the z-axis) via a rotational stage1004 while a translational stage 1006 simultaneously performstranslational movement in a pre-defined direction such that theelectrode head passes over a center of the object 103. This option isappropriate for objects 1002 with a circular shape (e.g., asemiconductor wafer). Different ratios of rotational and translationalrates can define a scanning path of an electrode array 1012 relative tothe surface and determine fineness of surface treatment. When an activepolishing mode is used, this ratio is aligned with the pixel size ofmetrology data. In addition, the translational movement of the stage canbe continuous or stepped and therefore defines the final scanningtrajectory of the object 1002. In some implementations, the design ofelectrode array 1010/1012 is such that the trajectory from eachelectrode will not overlap with the trajectory from any other electrode.

As shown in FIG. 10-1, for an object 1008 with a square or rectangleshape, the movement of the object 1008 is similar to that used in atypical linear scanning system. In this configuration, a 2-dimensionalelectrode array 1010 is easier to implement, compared with otherconfigurations.

If the object 1002/1008 has isolated regions (e.g., sparse regions) thatneed surface polishing, localized scanning is useful. In this mechanism,the object on the stage moves to the localized regions in a pre-definedorder and performs the polishing.

Electrode and Electrode Array Design

There are many potential materials suitable for “needle-type”electrodes, including: tungsten, copper, molybdenum, zirconium,tantalum, and/or combinations and alloys thereof. Other materials oralloys are also possible. To sustain a stable and durable manufacturingprocess for high volume manufacturing, the material is chosen such thatthe electrode is sufficiently capable of resisting electrical wear so asto withstand the high repetition rate of the discharging pulses and theaccompanying heat generation. In some implementations, a side surface ofthe electrode (e.g., to which the “needle-type” electrode tip isattached) is polished to a high degree of smoothness to avoid sharp tipsand/or valley. In some implementations, the shape of the electrode tipis a spherical or an otherwise smooth shape, rather than an acute angle.This is to avoid damage of the electrode tip via “explosion” dischargearound the acute angle.

In some implementations, to increase the polishing process throughputand improve efficiency, multiple “needle-type” electrodes can bearranged into a 1-dimensional array or 2-dimensional array. In someimplementations, all of the electrodes are identical and mutuallyexchangeable. In some implementations, the shape of each identicalelectrode tip is near “needle-type,” characterized by a non-acute angle(e.g., the electrode tip is characterized by an angle of 30 degrees witha spherical radius of curvature at the tip of 100-500 nanometers).According to the theory of gas discharge, the shape of the electrode tipinfluences the uniformity, stability, and shape of the dischargingpulse. When the electrode tip is sharp, electrons in the dischargingpulse have divergent angle so that the bombardment area on surface isspread out. Because the MFDP system works in a high gas pressureenvironment, maintaining a stable and slightly higher temperature on theelectrode array than in neighboring regions is helpful in terms ofkeeping the electrode clean and eliminating contamination generated fromthe discharging pulse.

In some implementations, the distance between electrodes is a constant.Alternatively, the distance between electrodes is different betweendiffering pairs of neighboring electrodes. In some implementations, thedistance between electrodes is a first constant distance in onedirection (e.g., the x-direction), and a second constant distance in adifferent direction (e.g., the y-direction), depending on, for example,the scanning mechanism design and object.

In some circumstances, it is suitable to individually control everyelectrode of an electrode array. This is because each electrode will beexposed to a different region of the surface with different metrologydata and therefore will be subject to a different discharge condition.

Library of Electrodes

In some implementations, the MFDP process is a high repetition ratedischarging process. The high repetition rate helps to meet high volumemanufacturing throughput demand. The repetition rate of the dischargingprocess can be from a few kilohertz (kHz) to tens of kilohertz and eachdischarge involves a short and intense release of energy from anelectrode tip. This may bring degradation or damage to the electrode tipand therefore reduce the surface polishing quality and/or cause systeminterruptions (e.g., system down time). Besides selection of anappropriate electrode material that can strengthen the electrode,real-time monitoring and adjustment of the electrode in the z-directionhelps to mitigation these issues. When an electrode is damaged or itsshape is changed so as to render it non-functional, or otherwise reachescriteria whereby the electrode cannot be further adjusted, a replacementof the electrode is performed. To improve system efficiency and reducesystem down time, a library of electrode arrays inside the maindischarge chamber is provided in some implementations to allow thesystem to quickly replace damaged electrodes and resume normalprocesses. FIG. 14 shows an example of an electrode library design (FIG.14-1 is a top view of the electrode library 1400 and FIG. 14-2 is a sideview of the electrode library 1400). In the example electrode librarydesign, all of the electrodes 100 are identical and ready-to-use.Furthermore, the electrodes 100 are arranged in a chain connectedformat. Each electrode 100 can be inserted into a chain or taken out bya robot. In some implementations, the position for electrode replacementis fixed for convenience, but other options are possible. After anelectrode 100 is replaced, the chain moves (e.g., using a libraryrotator 1402) and makes the next new electrode 100 available at thereplacement position. The status of the electrode library is recordedand shown in real-time and the library of electrodes can be replacedwhen no remaining electrodes 100 exists in it. Using this approach, auser does not have to place the system into a “down-state” for a singleelectrode replacement. This is useful because placing the system into a“down-state,” and more specifically bringing the system out of a“down-state” may, in some circumstances, involve complicated and timeconsuming operations, including vacuuming, air-balancing, partreplacement, high pressure gas re-filling, system balancing andre-calibration, and the like.

Calibration Panels and Adjustment

As shown in FIG. 11, in some implementations, outside of the polishingarea 1100 and on the stage, there are one or more calibration panels1102 (e.g., calibration panel 1102-1 and calibration panel 1102-2),which serve as a reference plane and surface standard. In someimplementations, there are multiple levels of calibration for thepolishing process. The calibration panels 1102 are manufactured with asufficiently high surface accuracy to meet the high accuracy polishingspecification (e.g., the calibration panel will have three times higheraccuracy, in terms of peak-valley error, than the polishingspecification). During calibration, a minimum allowable dischargedistance (or gap) between the electrode tip and the surface isestablished. For example, the minimum allowable discharge distance isset to a number in the range of 2-4 times a characteristic peak-valleyerror of the surface. In some circumstances, the calibrations panels1102 are calibrated and leveled before the system begins processing andcalibrated again whenever system is down for maintenance or repair work.In some implementations, the calibration is performed using a probe(e.g., an atomic force microscope or scanning electron microscope) toproduce a height map of the calibration panel 1102 and characterize atilt angle so that necessary adjustments can be made. After the systemis calibrated, the object is calibrated and/or adjusted with referenceto the calibration panels 1102. The calibration is performed usingpoints of a pre-defined grid on the surface 102 and, using thiscalibration, the system determines the reference or “z=0” plane of thesurface 102, by using either metrology data or an average value of agrid measurement. In some implementations, before the polishing processbegins, the electrode array 1010/1012 is calibrated against thecalibration panels 1102 to assure that all of the electrodes 100 in theelectrode array 1010/1012 are at a pre-determined height referenced tothe “z=0” plane. In some implementations, the surface 102 is dividedinto scanning paths, which can be bands for square region or zones forcircular region. In some implementations, the calibration is repeatedafter each scanning path is scanned, and the electrode tip is adjustedwith reference to the reference plane. For high accuracy polishing,real-time monitoring and adjustment can be performed during the processby moving the electrode array 1010/1012 back to the calibration panels,measuring the variation of the electrode tip height, and compensatingfor the change. In addition, the discharging pulses will generate heatand acoustic waves that can trigger chemical reactions near the tip ofthe electrode, which may wear or even damage the tip of electrode 100.For at least these reasons, the electrode tip height is adjustable inthe z-direction.

Energy Reservoirs for Charging and Discharging

In order to allow the system to continuously run the polishing processwith high stability and precision, some implementations provide anenergy reservoir to store electrical energy. Due to the fast throughputof objects through the process in a production environment, the systemgenerally maintains a high scanning speed of the stage and/or a highrepetition rate of discharging pulses. This helps to guaranteecontinuous operation without missing regions that need to be processed.To maintain the high speed of the scanning stage and/or the highrepetition rate of the discharging pulses with stable energy release,the stored electrical energy has a corresponding stable energy levelthroughout whole process. A conventional energy reservoir will havedifficulty meeting these requirements, especially when a large electrodearray is utilized.

To that end, the present invention provides a novel type of energyreservoir. In some implementations, the energy reservoir has acylindrically symmetric structure 1300, as shown in FIG. 13. In someimplementations, the energy reservoir has a structure that ischaracterized by another symmetry type (e.g., other than cylindrical).The energy reservoir is divided into a plurality of capacitor units andeach unit can independently support the entire electrode array. In someimplementations, each capacitor unit contains a plurality of electricalcapacitors for energy storage and each capacitor can independentlysupport a single electrode in the electrode array. While one capacitorunit is connected with the electrode array for discharging, the rest ofthe capacitor units can be undergoing a charging process. When thecharging speed in a respective capacitor unit that is connected to theelectrode array is insufficient to keep pace with a polishing scan(e.g., because of a low amount of charge on one or more capacitors inthe capacitor unit), the system can switch to a different, fullycharged, capacitor unit to continuing processing. As a generalprinciple, if the respective capacitor unit can support a kilohertz orhigher repetition rate of discharging pulses, the other capacitor unitwill remain idle with full charge.

Electrical Circuitry for Fast Discharging

Electrical circuitry and switching for the purposes of igniting andsustaining electrical discharge from a capacitor and supporting there-charging thereof is a mature technology which has been utilized invarious fields (e.g., high voltage short electrical current pulsegeneration, accelerator technology, Blumlein circuitry utilized inexcimer lasers, and so on). In principle, such circuitry uses capacitorsto store high voltage electrical energy and further utilizes a fasttriggering mechanism and circuitry with low inductance to realize fastdischarging. Conventional Blumlein fast discharge circuitry utilizes adischarge controlling switch. The discharge controlling switch ismaintained in an “off” position and both electrodes are held at an equalelectrical potential through an inductance connection while a chargingloop is charged to a high electrical voltage. A high pressure gasmixture inside the discharge chamber (and, therefore, betweenelectrodes) increases the breakdown voltage of the discharge cavity andthus steepens the shape and shortens the pulse-width of a resultantdischarging pulse. Several circuitry designs or configurations that cangenerate the fast discharge are known in the art. However, in terms ofthe discharge process, one difference between the present invention andpast applications is that, in the present invention, it is desirable toavoid energy excitation or an unwanted energy loss mechanism inside thedischarging medium. Therefore, gas species which have the potential toinduce excitation/emission or inelastic energy loss should be avoided sothat the majority of electrical discharge energy transfer can occurbetween the electrode and the surface.

In present invention, the polishing strength results from an electricfield with an electric field strength, E, which could be in range of10⁴-10⁶ V/cm or even higher. In some implementations, the distancebetween electrode and surface is quite small, so the charging voltagedoes not necessarily have to be very high (a range of 10²-10³ volts isenough for some practical processes). In some implementations, the fastdischarge process utilizes a high voltage but low electrical current. Tothis end, the electric field strength, E, is controlled below 10⁶ V/cm(in some circumstances, electric field strengths near or above a 10⁸V/cm level may generate a large electrical current, which could damagethe surface).

Preionization and Discharging Performance

The fast discharging process involves electrical breakdown and utilizesa steep rise of a discharging pulse. Adding a preionization processbefore the discharge can smooth the electric field formed in thedischarge cavity during a discharging pulse (e.g., reduce the divergenceof the electric field at various points in the discharge cavity).Consequently, the preionization process serves to ease the electricalbreakdown and gain controllability of discharging pulses. In the past,there have been various types of preionization processes (e.g. inexcimer lasers). In the present invention, a UV source lamp (or shortwavelength diode laser array) and micro-lens array can be utilized assource of preionization. In some implementations, lamp light is splitinto multiple beams and each beam passes through a transporting fiberand is focused by a micro-lens into the discharge cavity, near the tipof an electrode, shown in FIG. 6-3. In the case of a diode laser array,a laser diode and micro-lens can have one-to-one arrangement. A weakelectron beam generated by the preionization process plays a role in theformation and development of the discharging pulse and can contribute toshape of discharge path and the duration and rising time of thedischarging pulse. Therefore, the timing between preionization and therising edge of the discharging pulse and the duration and strength ofpreionization pulse can be developed as part of a polishing processrecipe. These values therefore comprise tuning parameters that a usercan use to enhance the flexibility, quality and capability of polishingprocess.

Layout of Multiple Discharge Chambers

In some implementations, a plurality of discharge chambers 702 (e.g.,discharge chamber 702-1, 702-2, 702-3, 702-4, and 702-5) are integratedinto a single MFDP system, as shown in FIG. 7. The plurality ofdischarge chambers 702, which each include an electrode head and stage,can be identical (e.g., for parallel processing of objects with the samespecifications), or can be different (e.g., for serial processing of anobject where each chamber is tuned to a different set of specifications,for example, to polish a different material or use different dischargeparameters and/or different processing condition). To increasethroughput in a manufacturing environment, this method provides enhancedcapacity for parallel polishing processes and flexibility for differentapplications and process flows. Multiple chambers 702 can be operated bya centralized control unit or by individual units. The objects can bestored in single storage unit or separate units, depending on thespecifics of the application. In the case of the identical processchambers for treatment of objects with similar incoming surface qualityand the same (or similar) specifications on surface accuracy, a largequantity output can be achieved by utilizing a single storage space andcentral control unit with high level automation.

Multiple MFDP Systems in Series

The minimum allowable discharging distance (or gap) between theelectrode tip and the reference plane (or z=0 plane) of the surface isdetermined based on a surface accuracy as measured by a maximumpeak-valley (P-V) error of surface. There is a maximum capability for asingle polishing process that is tied to the incoming surface accuracy.To achieve a high surface accuracy on a surface with an initially lowsurface accuracy (e.g., prior to polishing), a sequential polishingprocess is appropriate, in which the output object of a first MFDPsystem becomes the input object of a second MFDP system. The sameapproach follows if there are more than two systems in series. Forexample, as shown in FIG. 8, an object with a surface having an initialspecification labeled “spec #0,” is polished by an MFDP system 800-1 toa surface spec labeled “spec #1.” The object, now polished to spec #1,is then passed to an MFDP system 800-2 to a surface spec labeled “spec#2.” The process proceeds in an analogous manner through MFDP systems800-3 through 800-x (which can include an arbitrary number of MFDPsystems).

It is appropriate to set a polishing ratio for a single MFDP system (apolishing ratio as used herein means the ratio of an output surfaceaccuracy and to an input surface accuracy). As an example, one can use amaximum P-V value as the measureable quantity for surface accuracy andset the ratio in the range of 25%-40%. As another example, a 30% ratiocan be used as a starting point for process engineering and equipmentdevelopment. Use of a fixed polishing ratio is advantageous for objectalignment and electrode tip calibration and for reducing the risk ofelectrode damage or degradation of the electrode shape around the sharptip.

Discharging Medium Cycling and Filtering

FIG. 12 shows a processing gas flowing loop. The loop includes the gaschamber 302, a vacuum system 1200 for providing vacuum conditions andremoving gases from the gas chamber 302, a filter 1202 for cleaninggases for reuse, a feeding/refilling unit 1204 that provides couplingbetween the various gas lines and a gas storage system 1206, and a flowcontrol 1208 for controlling gas flow into the gas chamber 302. Therestoration time of the discharging medium after a discharging pulse isin range of nanosecond to tens of nanoseconds, which is sufficient tosupport kilohertz or higher discharge repetition rates. However, duringthe process, the discharging mixture can be contaminated. To improve thequality of the discharging pulses and remove contamination inside thedischarging medium, cycling and filtering is performed in accordancewith some implementations. Moreover, in some implementations, usage ofthe discharging medium through cycling can last a long time withoutre-filling because the discharging medium is stable. In the polishingprocess, the discharging medium plays a role for breakdown thresholdcontrol and the transfer of electrical energy to the surface. Thus, somegases, especially those with lower excitation energy levels, should beavoided. The gas flowing direction should be chosen to minimizevibrations of the electrode and/or the electrode array, as well asminimize force on electrodes.

Remaining Charge on the Surface after Polishing

Due to the nature of micro-fast discharging polishing, some electricalcharge may remain on the surface after the polishing process, even ifthe object is grounded before and during the polishing process. Toremove the residual electrical charge from the surface, according tosome implementations, there is a buffering period after polishing,during which the object is grounded for a certain period of time.

While the electric field strength, E, is extremely strong between theelectrode tip and the surface during discharging, the voltage appliedbetween the electrode and the surface is typically in the range ofhundreds of volts because the discharge distance between the electrodetip and the surface is quite small (e.g., in the range ofsub-millimeters or tens of micrometers or even smaller). Therefore, thevoltage due to remnant charge on surface after discharge polishing iswell under a thousand volts, which is the critical threshold forelectrostatic triggered surface damage, although the remaining voltageon surface needs to be eliminated.

Post-Processing

The fast discharge polishing process will crash surface “hills” andgenerate debris. While some debris is removed by flowing of thedischarging medium, some debris may remain on the surface, and thereforeneeds to be removed. Meanwhile, the discharge process may cause someelectrical charge to accumulate on the surface even though the object isgrounded during processing, which will create unwanted effects on thepolished object and impact the subsequent process steps. These issuesplus others, e.g. non-uniform heat accumulation or stress distribution,are dealt with in post-processing. After the polishing process is doneand the object is moved out of the chamber, it can be transferred to aseparated unit for post-processing, before it returns to the objectstorage library. Debris clean-up can be done using pressurized dry airflow at a grazing incidence relative to the surface. To remove residualelectrical charge from polished surface, one can reverse the polarity ofelectrical connection. In this case, the object is still grounded but apad with positive electrical polarity touches an edge or outer area ofthe object.

Also, optionally, during post-processing, is an operation to releasesurface stress. For high accuracy surface polishing at sub-micrometer ornanometer levels, any remaining surface stress accumulated from thepolishing process or non-uniform heating generated by the dischargepolishing process may induce surface distortions and therefore impactthe final quality of the fast discharge polishing. To reduce oreliminate remaining stress, the object can be warmed up for a certainperiod of time and then gradually cooled down. This post-processing issimilar to those familiar processes for material heat treatment orannealing, and this step can release both surface stress and non-uniformheat accumulation across surface.

Environmental Controls

Several environmental controls are provided that achieve various tasks,optionally including: electromagnetic field shielding and grounding,temperature controls, vibration isolation, and acoustic noise isolation.All of requirements for environmental control of the MFDP system, exceptacoustic noise isolation, should be similar to those of a typicaloptical or e-beam imaging/lithography system or other sophisticatedprocess equipment. The acoustic noise control is to shield the noisegenerated by high repetitive discharging process so that is will notinterfere with the neighboring equipment.

Methods of Surface Polishing

FIGS. 15-1 and 15-2 are flow diagrams illustrating a method 1500 ofpolishing a surface, in accordance with some implementations.

The method 1500 includes generating (1502) a pixel map of a surface. Thepixel map includes a plurality of pixels including a first pixel and asecond pixel. The first pixel corresponds to a first surface errorassociated with a first location on the surface and the second pixelcorresponds to a second surface error associated with a second locationon the surface.

In some implementations, the pixel map is generated (1504) in real-timeusing a surface height measurement sensor (e.g., using an opticalinterferometer, height sensor, or electromagnetic probe incorporatedinto the MFDP system). Alternatively, the pixel map is generated (1506)using a metrology tool (e.g., independent of the MFDP system) prior tothe filling, positioning, and determining operations, which aredescribed below (cf. operations 1508, 1510, and 1516). Stillalternatively, in some implementations, the pixel map is generated usinga metrology tool that is incorporated into the MFDP system (e.g., priorto the positioning operation 1510).

The method 1500 further includes filling (1508) the gas chamber with adischarging medium to a predefined pressure. For example, in someimplementations, the discharging medium includes helium, or nitrogen,and/or mixture thereof and the predefined pressure is in a range of oneatmosphere to tens of atmospheres.

The method 1500 further includes positioning (1510) an electrode withrespect to the surface such that the electrode is proximal to the firstlocation. In some implementations, the electrode is (1512) a respectiveelectrode in an electrode array. The electrode array includes aplurality of electrodes. It should be understood that positioning theelectrode with respect to the surface can be achieved in any number ofways, including: maintaining the electrode at a fixed position whilemoving the surface (e.g., the object having the surface), maintainingthe surface at a fixed position while moving the electrode, or movingboth the surface and electrode. In other words, positioning theelectrode with respect to the surface should be construed to mean anytype of mechanical movement that results in the stated relative positionof the surface and the electrode.

In some implementations, the electrode is (1514) a needle-type electrodehaving a tip. The tip has a distal end disposed proximal to the surfaceand characterized by a radius of curvature at the distal end within afirst predefined range, and an included angle within a second predefinedrange. The internal angle, denoted α, is given by the formula α=2π−β,where β is the outer angle described with reference to FIGS. 1-1 and1-2. In some implementations, the first predefined range is one of thegroup consisting of: 10 nm to 100 nm, 50 nm to 500 nm, and 100 nm to2000 nm; and the second predefined range is one of the group consistingof: 15 degrees to 20 degrees, 5 degrees to 45 degrees, and 10 degrees to30 degrees.

The method 1500 further includes determining (1516) if the first surfaceerror meets predefined polishing criteria. In accordance with adetermination that the first surface error meets (1518) the predefinedpolishing criteria, the method 1500 includes triggering an electricalbreakdown of the discharging medium whereby the electrical breakdownresults in a discharging pulse that polishes the surface.

In some implementations, triggering the electrical breakdown includes(1520) applying a voltage between the electrode and the surface. Thevoltage is greater than a breakdown voltage of the discharging medium.In some implementations, the application of the voltage between theelectrode and the surface is gated (1522) so as to control a temporalduration of the discharging pulse. In some implementations, theapplication of the voltage between the electrode and the surface isgated using a gas-filled tube (e.g., a gas-filled tube used as a fastswitch). Alternatively, a different type of switch with a fast responsetime is used instead of a gas-filled tube.

In some implementations, triggering the electrical breakdown includesapplying (1524) a preionization signal to a region between the electrodeand the surface. In some implementations, the preionization signal isprovided (1526) by one of a laser (meaning one or more lasers) or anultraviolet (UV) lamp. In some implementations, lamp or laser light(e.g., from the UV lamp or from the one or more lasers, respectively) issplit into multiple beams and each beam passes through a transportingfiber and is focused by a micro-lens into the discharge cavity, near thetip of an electrode (e.g., a respective electrode in an electrodearray).

In accordance with a determination that the first surface error does notmeet the predefined polishing criteria (1528), the method 1500 furtherincludes forgoing (1530) triggering of the electrical breakdown of thedischarging medium and re-positioning (1532) the electrode with respectto the surface such that the electrode is proximal to the secondlocation.

It should be understood that the particular order in which theoperations in FIGS. 15-1 and 15-2 have been described is merelyexemplary and is not intended to indicate that the described order isthe only order in which the operations could be performed. One ofordinary skill in the art would recognize various ways to reorder theoperations described herein.

FIGS. 16-1 and 16-2 are flow diagrams illustrating a method 1600 ofpolishing a surface, in accordance with some implementations.

The method 1600 includes applying (1602) a voltage between an electrodeand a surface. In some implementations, the electrode is (1604) aneedle-type electrode having a tip. The tip has a distal end disposedproximal to the surface and characterized by a radius of curvature atthe distal end within a first predefined range, and an included anglewithin a second predefined range. The internal angle, denoted α, isgiven by the formula α=2π−β, where β is the outer angle described withreference to FIGS. 1-1 and 1-2. In some implementations, the firstpredefined range is one of the group consisting of: 10 nm to 100 nm, 50nm to 500 nm, and 100 nm to 2000 nm; and the second predefined range isone of the group consisting of: 15 degrees to 20 degrees, 5 degrees to45 degrees, and 10 degrees to 30 degrees. In some implementations, theelectrode is (1606) a respective electrode in an electrode array. Theelectrode array includes a plurality of electrodes.

The method 1600 further includes calibrating (1608) a height of theelectrode relative to the surface so as to establish electricalbreakdown threshold criteria. In some circumstances, the result of thecalibration operation 1608 is that the electrode tip is positioned at afirst height with respect to an average height of the surface. In suchcircumstances, the applied voltage at the first height in insufficientto cause electrical breakdown (e.g., the applied voltage is less thanthe breakdown voltage). However, the surface may include locations witha corresponding height such that the applied voltage is sufficient tocause electrical breakdown, thereby releasing a discharging pulse, asdescribed below with reference to operation 1616.

In some implementations, the method 1600 includes applying (1610) apreionization signal to a region between the electrode and the surface(e.g., the discharge cavity). In some implementations, the preionizationsignal is provided 1526) by one of a laser (meaning one or more lasers)or an ultraviolet (UV) lamp. In some implementations, lamp or laserlight (e.g., from a UV lamp or from one or more lasers, respectively) issplit into multiple beams and each beam passes through a transportingfiber and is focused by a micro-lens into the discharge cavity, near thetip of an electrode (e.g., a respective electrode in an electrodearray).

The method 1600 further includes scanning (1614) the electrode withrespect to the surface so as to sequentially position the electrode overa plurality of locations on the surface, each location characterized bya surface error. It should be understood that scanning the electrodewith respect to the surface can be achieved in any number of ways,including: maintaining the electrode at a fixed position while movingthe surface (e.g., the object having the surface), maintaining thesurface at a fixed position while moving the electrode, or moving boththe surface and electrode. In other words, scanning the electrode withrespect to the surface should be construed to mean any type ofmechanical movement that results in a change in the relative position ofthe surface and the electrode.

In some circumstances, the surface error for each location in theplurality of locations includes (1616) a surface height. The electricalbreakdown threshold criteria are then met when the surface height issuch that a distance between the electrode and the surface causes thevoltage to exceed a breakdown voltage of the discharging medium.

In any event, when a respective location in the plurality of locationshas a surface error that meets the electrical breakdown thresholdcriteria, electrical breakdown occurs (1618), whereby the electricalbreakdown results in a discharging pulse that polishes the surface. Insome implementations, the application of the voltage is gated (1620) soas to control a temporal duration of the discharging pulse. Theapplication of the voltage is gated using (1622) a gas-filled tube, oranother type of switch with a fast response time.

It should be understood that the particular order in which theoperations in FIGS. 16-1 and 16-2 have been described is merelyexemplary and is not intended to indicate that the described order isthe only order in which the operations could be performed. One ofordinary skill in the art would recognize various ways to reorder theoperations described herein.

FIG. 17 is a process diagram illustrating a semiconductor process 1700in which MFDP polishing is used, in accordance with someimplementations. In some implementations, semiconductor process 1700 isused in conjunction with other known processes to fabricate asemiconductor device on a chip.

Process 1700 optionally includes a metallization operation 1702 and aninsulator growth operation 1704. Collectively, the metallizationoperation 1702 and the insulator growth operation 1704 serve tofabricate a respective layer of the semiconductor device. In somecircumstances, the layer may include features such as a vias, contacts,interconnects, source and drain regions of field effect transistors,metallic structures, microelectromechanical system (MEMS) structures,and the like. In some implementations, known operations other thanmetallization and insulator growth may be substituted for one or both ofoperations 1702 and 1704, respectively. In some implementations, theprocess 1700 may include multiple insulator growth operations 1704, eachof which operation includes, for example, patterning and etching.

After the layer is deposited, the process 1700 optionally includeschemical mechanical polishing operation 1706, which can be performedusing conventional chemical mechanical polishing (CMP) methods known inthe art.

To improve upon the result of CMP operation 1706, the chip issubsequently polished using an MFDP operation 1708. If the surfacespecification is not satisfactory (1710—No) following the MFDP operation1708, another MFDP operation 1708 is performed. In some circumstances,subsequent MFDP operations 1708 are performed using the same MFDP systemtuned to different parameters, while in some implementations, subsequentMFDP operations 1708 are each performed using a different MFDP system(e.g., as described with reference to FIG. 8).

If the surface specification is satisfactory (1710—Yes), and there areno more device layers to be fabricated (1712—No), then the process 1700is complete (1714). On the other hand, if more layers are to befabricated (1712—Yes), then the process 1700 begins again atmetallization operation 1702.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific implementations. However, theillustrative discussions above are not intended to be exhaustive or tolimit the implementations to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The implementations were chosen and described in order tobest explain the principles of the disclosure and its practicalapplications, to thereby enable others skilled in the art to bestutilize the various implementations with various modifications as aresuited to the particular use contemplated.

It will be understood that, although the terms “first,” “second,” etc.are sometimes used herein to describe various elements, these elementsshould not be limited by these terms. These terms are only used todistinguish one element from another. For example, a first element couldbe termed a second element, and, similarly, a second element could betermed a first element, without changing the meaning of the description,so long as all occurrences of the “first element” are renamedconsistently and all occurrences of the second element are renamedconsistently. The first element and the second element are bothelements, but they are not the same element.

The terminology used herein is for the purpose of describing particularimplementations only and is not intended to be limiting of the claims.As used in the description of the implementations and the appendedclaims, the singular forms “a,” “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “comprises” and/or “comprising,” when used inthis specification, specify the presence of stated features, integers,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers,operations, elements, components, and/or groups thereof

As used herein, the term “if” may be construed to mean “when” or “upon”or “in response to determining” or “in accordance with a determination”or “in response to detecting,” that a stated condition precedent istrue, depending on the context. Similarly, the phrase “if it isdetermined (that a stated condition precedent is true)” or “if (a statedcondition precedent is true)” or “when (a stated condition precedent istrue)” may be construed to mean “upon determining” or “in response todetermining” or “in accordance with a determination” or “upon detecting”or “in response to detecting” that the stated condition precedent istrue, depending on the context.

Throughout the preceding description, various implementations aredescribed within the context of wafers, chips, reticles and the like.This is purely for convenience of explanation and is not intended tolimit the claims that follow.

What is claimed is:
 1. A method of polishing a surface of an objectdisposed within a gas chamber, comprising: filling the gas chamber witha discharging medium to a predefined pressure; applying a voltagebetween an electrode and the surface; calibrating a height of theelectrode relative to the surface so as to establish one or moreelectrical breakdown threshold criteria; and scanning the electrode withrespect to the surface so as to sequentially position the electrode overa plurality of locations on the surface, each location characterized bya surface error; wherein when a respective location in the plurality oflocations has a surface error that meets the electrical breakdownthreshold criteria, electrical breakdown occurs; wherein the electricalbreakdown results in a discharging pulse that polishes the surface. 2.The method of claim 1, wherein: the surface error for each location inthe plurality of locations includes a surface height; the electricalbreakdown threshold criteria are met when the surface height is suchthat a distance between the electrode and the surface causes the voltageto exceed a breakdown voltage of the discharging medium.
 3. The methodof claim 1, wherein the application of the voltage is gated so as tocontrol a temporal duration of the discharging pulse.
 4. The method ofclaim 3, wherein the application of the voltage is gated using agas-filled tube.
 5. The method of claim 1, further including applying apreionization signal to a region between the electrode and the surface.6. The method of claim 5, wherein the preionization signal is providedby one of a laser or an ultraviolet lamp.
 7. The method of claim 1,wherein the electrode is a needle-type electrode having a tip, said tiphaving a distal end disposed proximal to the surface and characterizedby: a radius of curvature at the distal end within a first predefinedrange; and an included angle within a second predefined range.
 8. Themethod of claim 7, wherein: the first predefined range is one of thegroup consisting of: 10 nm to 100 nm, 50 nm to 500 nm, and 100 nm to2000 nm; and the second predefined range is one of the group consistingof: 15 degrees to 20 degrees, 5 degrees to 45 degrees, and 10 degrees to30 degrees.
 9. The method of claim 1, wherein the electrode is arespective electrode in an electrode array, the electrode arrayincluding a plurality of electrodes.
 10. An apparatus for polishing asurface of an object disposed within a gas chamber, comprising: anelectrode; a gas chamber, wherein the object is disposed within the gaschamber; a scanning stage configured to position the electrode withrespect to the surface; a gas inlet system configured to fill the gaschamber with a discharging medium to a predefined pressure; a powersupply configured to apply a voltage between the electrode and thesurface; a computer system that includes one or more processors, memory,and one or more programs stored in the memory, the one or more programscomprising instructions that when executed by the one or more processorscause the computer system to: calibrate a height of the electroderelative to the surface so as to establish one or more electricalbreakdown threshold criteria; and instruct the scanning stage to scanthe electrode with respect to the surface so as to sequentially positionthe electrode over a plurality of locations on the surface, eachlocation characterized by a surface error; wherein when a respectivelocation in the plurality of locations has a surface error that meetsthe electrical breakdown threshold criteria, electrical breakdownoccurs; wherein the electrical breakdown results in a discharging pulsethat polishes the surface.
 11. The apparatus of claim 10, wherein: thesurface error for each location in the plurality of locations includes asurface height; the electrical breakdown threshold criteria are met whenthe surface height is such that a distance between the electrode and thesurface causes the voltage to exceed a breakdown voltage of thedischarging medium.
 12. The apparatus of claim 10, wherein theapplication of the voltage is gated so as to control a temporal durationof the discharging pulse.
 13. The apparatus of claim 12, wherein theapplication of the voltage is gated using a gas-filled tube.
 14. Theapparatus of claim 10, further including one of a laser or anultraviolet lamp configured to apply a preionization signal to a regionbetween the electrode and the surface.
 15. The apparatus of claim 10,wherein the electrode is a needle-type electrode having a tip, said tiphaving a distal end disposed proximal to the surface and characterizedby: a radius of curvature at the distal end within a first predefinedrange; and an included angle within a second predefined range.
 16. Theapparatus of claim 15, wherein: the first predefined range is one of thegroup consisting of: 10 nm to 100 nm, 50 nm to 500 nm, and 100 nm to2000 nm; and the second predefined range is one of the group consistingof: 15 degrees to 20 degrees, 5 degrees to 45 degrees, and 10 degrees to30 degrees.
 17. The apparatus of claim 10, wherein the electrode is arespective electrode in an electrode array, the electrode arrayincluding a plurality of electrodes.